Nanomaterials are characterized in particular physical/chemical properties such as small size effect, surface effect, quantum tunneling effect, etc. Metal nanomaterials become important catalysts because of the excellent properties. Commonly used metal catalysts include generally platinum metals, coinage metals, ferric metals, etc., and are widely used for energy conversion, petroleum chemistry, purification of exhaust gas of automobiles and chemical industries. It has been always an important issue in the field for how to improve the metal nanocatalyst in the activity, selectivity, stability and use efficiency. Taking platinum metal catalyst as an example, a study employing platinum single crystal surface as a model catalyst exhibits that, performance of the catalyst depends on the catalyst's surface structure. A high-index crystal surface having an open structure and high surface energy has the catalytic activity and stability remarkably advantageous over a low-index crystal surface whereon atoms are tightly lined up (see reference document: [1] Na Tian, Zhi-You Zhou, Shi-Gang Sun, Platinum Metal Catalysts of High-Index Surfaces: From Single-Crystal Planes to Electrochemically Shape-Controlled Nanoparticles. J. Phys. Chem. C., 2008, 112: 19801-19817). Fundamental studies on other metal catalysts gave the similar conclusion, i.e., the surface structure is the decisive factor of performance of the metal catalysts, and a catalyst having an open surface structure has a higher activity and stability. Further, different surface structures may have particular catalytic property with respect to special reaction, i.e., exhibiting catalytic selectivity of surface structure. Currently, commercially available metal nanocatalysts are particles or crystals of several nanometers in size, and their surfaces have a structure of crystal planes with compact atomic arrangement. As the surface structure of a nanocrystal is determined by the shape of the nanocrystal, changing the shape of the prepared nanocrystal can change the surface structure, and thus control the activity and selectivity of the metal nanocatalyst at the level of atomic arrangement structure.
First of all, depositing platinum nanospheres on surface of glassy carbon electrodes, then appling square wave potential treatment to the surface to dissolve the platinum sphere and allow nucleation growth again. With respect to this, the present applicant ([2] Na Tian, Zhi-You Zhou, Shi-Gang Sun, Yong Ding, Zhong Lin Wang, Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity. Science, 2007, 316: 732-735; [3] Chinese patent ZL 2007 1 0008741.4, platinum tetrahexahedral nanocrystal catalyst and preparation and use thereof) has prepared successfully the tetrahexahedral platinum nanocrystal catalyst, which has an activity 2-4 times of the commercially available platinum nanocatalyst. Further employing the process of direct electro-deposition, the present applicant ([4] Na Tian, Zhi-You Zhou, Neng-Fei Yu, Li-Yang Wang, Shi-Gang Sun, Direct Electrodeposition of Tetrahexahedral Pd Nanocrystals with High-Index Facets and High Catalytic Activity for Ethanol Electrooxidation, J. Am. Chem. Soc. 2010, 132: 7580-7581) further prepared palladium tetrahexahedral nanocrystal catalyst. The study results show that the platinum nanocatalyst having an open surface structure has a high density of active site, thus improving the catalytic activity remarkably. The present applicant ([5] Zhi-You Zhou, Zhi-Zhong Huang, De-Jun Chen, Qiang Wang, Na Tian, and Shi-Gang Sun, High-Index Faceted Platinum Nanocrystals Supported on Carbon Black as Highly Efficient Catalysts for Ethanol Electrooxidation, Angew. Chem. Int. Ed. 2010, 49: 411-414.) mixes platinum precursor and carbon black, and drops the mixture onto surface of glassy carbon electrodes, and carries out a square wave potential treatment to prepare a carbon-supported high-index faceted platinum nanocatalyst, further improving the utilization efficiency of platinum. The present applicant ([6] Yan-Xin Chen, Sheng-Pei Chen, Zhi-You Zhou, Na Tian, Yan-Xia Jiang, Shi-Gang Sun, Yong Ding, Zhong Lin Wang, Tuning the Shape and Catalytic Activity of Fe Nanocrystals from Rhombic Dodecahedral and Tetragonal Bipyramids to Cubes by Electrochemistry, J. Am. Chem. Sco. 2009, 131: 10860-10862) further prepares iron nanocrystal catalysts having various shapes such as rhombic dodecahedron, tetragonal bipyramids, 18-facet polyhedra, and cube on surface of glassy carbon electrodes with electrochemical method, which have very high electro-catalytic activity with respect to the reduction of nitrite. Studies also reveal the rule that more open of the surface structure of the metal catalyst, the higher the catalytic activity is.
It is worthy of pointing out that the above metal nanocatalysts having an open surface structure all grow on surface of glassy carbon electrodes, form at most one layer of metal nanocrystal with very few amount, and can hardly be used to practical catalytic system and industrial processes. There is yet no report about the preparation technology of adding metal precursor to the flowing liquid-phase reaction solution as a metal source, and the metal is controlled for nucleation first and growing later with the programmed potential.